The term waferboard as used throughout this disclosure is intended to include conventional waferboard, oriented strand board, oriented long wafer products, particleboard, fibreboard, flakeboard, parallel strand lumber products, composite lumber or the like.
It is common practice in conventional heated platen pressing (no steam applied directly to the layup) of waferboard to use either a liquid or a powdered phenolic resin adhesive as the binder as each are quite satisfactory for the purpose. The combination of both a liquid and a powder applied in sequence as the adhesive has also been used in conventionally heated platen pressing of waferboard.
U.S. Pat. No. 3,968,308 issued Jul. 6, 1976 to Buschfield et al describes a process of applying powdered adhesive through a liquid spray in order to adhere the powdered adhesive to the chips. This patent discusses the prior art attempts to solve the problem by moistening the chips prior to binder addition by spraying water on the chips or using chips with high residual moisture particularly in the centre layers or simultaneously moistening an application of powder resin.
The concept of steam pressing to consolidate particleboard is well known and is used commercially. The use of phenol formaldehyde resin for bonding steam pressed particle board is described in an article entitled Steam Press Process for Curing Phenolic-Bonded Particleboard, Forest Products Journal, Volume 23, No. 3, March 1973 by Shen. In this article a description is given of a process of consolidating hardwood particles using a liquid phenol formaldehyde resin and the application of this technique to produce boards having significantly better dimensional stability. Similar studies were carried out by Geimer (Steam Injection Pressing, proceedings of the 16th Washington State University International Symposium on Particleboard, 1982, Mar. 30 and Apr. 1, Pullman, Washington, Geimer et al (see Thick Composite are Technically Feasible with Steam-Injection Pressing' presented at Composite Board Product for Furniture and Cabinets: Inventions in Manufacture and Utilization, Greensboro, N.C., Nov. 11-13, 1986 and Steam Injection Pressing-Large Panel Fabrication with Southern Hardwoods in Proceedings of the 20th International Particleboard/Composite Materials Symposium; Apr. 8-10, 1986, Pullman, Washington.)
Despite the indication by Shen and Geimer et al that phenolic resins could be used in binding of flake boards and the like under steam pressing conditions, the art has found that the use of phenol formaldehyde resins in steam pressing is generally unsatisfactory (also described in the above identified publication Steam Injection Pressing-Large Panel Fabrication with Southern Hardwoods by Geimer, Apr. 1986) and Steam Injection Pressing, Kamke et al, FPRS 45th Annual Meeting, New Orleans, Louisiana, Jun. 1991. Generally it has been found that the internal bond of consolidated products made using phenolic resins in a steam press is simply too low or inconsistent and have recently been reported as less than 50 psi (Phenolic Resin Interaction During Steam-Injection Pressing of Flakeboard by Kamke et al and Use of Phenol-Formaldehyde Resin in Steam Pressing by Hsu, Adhesives & Bonded Wood Symposium, Seattle, Washington, Nov. 19-21, 1991).
Various steam pressing cycles have been advanced to consolidate particle board as shown for example in U.S. Pat. No. 4,517,147 issued May 14, 1985 to Taylor et al or U.S. Pat. No. 4,684,489 issued Aug. 7, 1987 to Walter.
It has also been suggested by Hickson in U.S. Pat. No. 4,937,024 issued Jun. 26, 1990 using a steam pressing technique and wherein esters in gaseous form is injected into the mat at final density to cure at least a portion of the phenol formaldehyde binder.
Generally the resin used in bonding of steam pressed waferboards and the like is an isocyanate type resin which has much more tolerance to moisture thereby facilitating the formation of a consolidated board and curing of the resin.
The costs of isocyanate resins are however, significantly higher than those of phenol formaldehyde resins and thus it would be advantageous to provide a system permitting the use of phenol formaldehyde based resins as the bonding agent for steam pressing of waferboards as opposed to the isocyanate resins now used commercially.